What is the fastest way to determine whether a string is a number?

Hi, I have a string x and in order to determine wether it is a number one can do the following

try
 flag = isnumeric(eval(x));
catch
  flag=false;
end

Is there a better and faster way to do this?

Thanks

Pat.

Answer 1

[Sean de Wolski]

Probably:

isnan(str2double(str))

This won't need try/catch either.

Anything with eval in it will definitely not be fast.

[Patrick Mboma]

Hi Sean, Your solution does work and is superior to mine.

thanks,

Pat.

Answer 2

[Cedric]

The following should be quite fast (faster than STR2DOUBLE for example):

 flag = ~isempty( sscanf( x, '%f' )) ;

EDIT: if characters can appear only at the end of the string, the following will be very fast

 flag = x(end) >= '0' && x(end) <='9'

[Patrick Mboma]

Hi Cedric,

I have tried ~isempty( sscanf('.03f', '%f' )) and it gave the wrong answer

[Cedric]

Then make your question more specific by indicating the type of strings that you can get. E.g. can you have floats or are these integers only, are characters at then end only, etc.

[Jos (10584)]

str2double involves a lot of ifs to cope with the many ways strings can contain valid numbers in matlab. Moreover, the numbers are then read in using sscanf. Take a look at

type str2double.m

Most likely, you can restrict your case to only one of these and directly use the appropriate sscanf version, as Cedric suggests.

[Patrick Mboma]

Hi Jos(10584),

There is definitely something to what Cedric suggests but I do not know a priori what the string will contain. All I need to know is whether the whole string is a number or not. If there is a way of restricting the number of cases that str2double will investigate, I am all ears. Thanks.

[Cedric]

Can these numbers be floating point or are they integers only? Can they be negative?

Most important, do you really need to optimize further than STR2DOUBLE? The latter is certainly the most straightforward approach; most alternatives would be tricks based on some assumption(s).

For example, if you are only dealing with positive integers, the following will be quite fast

 flag = all(x >= '0') && all(x <= '9') ;

[Patrick Mboma]

Thanks Cedric, in principle, the strings can be anything: floats, negative numbers, complex, etc. The bottom line is that if a string can be evaluated to give a double, then I separate it from the rest. In that case, although str2double goes through lots of checks, it seems superior to what I did earlier